A typically known laser-frequency stabilizer is adapted to change a resonator length thereof based on a saturated absorption line included in an optical output signal obtained by irradiating an absorption cell with a laser beam to stabilize an oscillation frequency of the laser beam to a specific saturated absorption line (see, for instance, Patent Literature 1: JP-A-2013-16713).
FIG. 5 is a block diagram showing a typical laser-frequency stabilizer 100.
As shown in FIG. 5, the laser-frequency stabilizer 100 includes a laser generator 10, a laser-beam detector 20 and a drive control unit 30.
The laser generator 10 includes an excitation semiconductor laser 11 configured to emit a laser beam L1 of 808 nm wavelength, and a resonant-wave generator 12 configured to receive the laser beam L1 and output a laser beam L2 of 532 nm wavelength.
The resonant-wave generator 12 includes optical components such as Nd:YVO4 crystal 121 configured to emit a light of 1064 nm wavelength by stimulated emission, a KTP crystal (non-linear optical crystal) 122 configured to convert a part of the light of 1064 nm wavelength into a light of 532 nm wavelength, an etalon 123 configured to transmit only a specific frequency of the laser beam, and a reflection mirror 124 configured to reflect the light of 1064 nm wavelength and transmit the light of 532 nm wavelength, and a resonator casing 125 housing the optical components.
The etalon 123 disposed in the resonator casing 125 provides the laser beam L2 in a single-mode.
Further, an actuator 126 (e.g. piezoelectric element) configured to change the position of the reflection mirror 124 (i.e. change the resonator length) in accordance with the applied voltage is disposed inside the resonator casing 125.
In the laser-beam detector 20, the laser beam L2 transmitted through a λ/2 plate 21 is divided by a first polarization beam splitter 22 into a laser beam L3 used for length measurement and the like and a laser beam L4 used in below-described saturated absorption line search process (referred to as a “search process” hereinafter) and a laser beam oscillation frequency locking process (referred to as a “frequency locking process” hereinafter).
Further, in the laser-beam detector 20, after the laser beam L4 passes through a second polarization beam splitter 23, a quarter-wave plate 24, and an iodine cell (absorption cell) 25, the laser beam L4 is reflected by the reflection mirror 26 toward the iodine cell 25.
Furthermore, in the laser-beam detector 20, after passing through the iodine cell 25 and the quarter-wave plate 24, the laser beam L4 is reflected by the second polarization beam splitter 23 toward a light detector 27 (converter), at which the laser beam L4 is photoelectrically converted to output an optical output signal S1.
FIGS. 6A and 6B show the optical output signal S1 and a secondary differentiation signal S2.
It should be noted that an ordinate axis in FIG. 6A represents output values of the signals S1, S2 and abscissa axis represents an output voltage V to the actuator 126, thereby showing the waveforms of the signals S1, S2 in accordance with the change in the output voltage V (i.e. in accordance with the change in the resonator length). FIG. 6B shows the secondary differentiation signal S2 in an area Ar of FIG. 6A in an enlarged manner.
As shown in FIG. 6A, when the output voltage V is varied over a wide range, it is understood that absorption lines M1 to M4 (referred to as peak groups M1 to M4 for the convenience of description hereinafter) periodically appear. It should be noted that the peak group M1 is the same peak group as the peak group M3, and the peak group M2 is the same peak group as the peak group M4.
The peak groups M1 to M4 are bundles of saturated-absorption-line groups. For instance, as shown in FIG. 6B, the peak group M2 is provided by (in an ascending order of the output voltage V) a saturated-absorption-line group N1 (a saturated absorption line a1), a saturated-absorption-line group N2 (saturated absorption lines a2 to a5), a saturated-absorption-line group N3 (saturated absorption lines a6 to a9), a saturated-absorption-line group N4 (a saturated absorption line a10), a saturated-absorption-line group N5 (saturated absorption lines a11 to a14), and a saturated-absorption-line group N6 (a saturated absorption line a15).
The drive control unit 30 controls an operation of the actuator 126 based on the optical output signal S1 (i.e. adjust the resonator length) to stabilize the oscillation frequency to a specific one of the saturated absorption lines.
Specifically, the drive control unit 30 includes an actuator controller 32 configured to control an actuator drive circuit 33 based on a control signal of the control unit 31 (i.e. adjust a voltage value V′ outputted to the actuator drive circuit 33) to change the output voltage V to the actuator 126.
It should be noted that the drive control unit 30 includes modulation/demodulation signal generator 34 configured to output signals of frequencies 1f, 2f and 3f Hz, a secondary differentiation lock-in amplifier 35 and third differentiation lock-in amplifier 36 (generators) configured to modulate the optical output signal S1 obtained by excitation of the laser beam L2 modulated by the actuator drive circuit 33 based on the signal of 1f Hz frequency using the frequencies 2f and 3f Hz to generate the secondary differentiation signal S2 and the third differentiation signal S3 respectively in addition to the above-described actuator controller 32, the actuator drive circuit 33 and the control unit 31.
The control unit 31 once measures a saturated absorption line (i.e. measures the number of the saturated-absorption-line groups belonging to each of the peak groups M1 to M4 and the number of the saturated absorption line(s) belonging to each of the saturated-absorption-line groups) in the search process, and again measures the saturated absorption line in the frequency locking process to lock the oscillation frequency to a desired one of the saturated absorption lines.
Herein, when the control unit 31 identifies the saturated absorption lines, the control unit 31 determines the saturated absorption lines after removing noise components based on the optical output signal S1 and the secondary differentiation signal S2 of the optical output signal S1 (see Patent Literature 1). With the above arrangement, even when there is an originally non-observable peak (noise) between the peak groups, between the saturated-absorption-line groups in the same peak group or between the saturated absorption lines in the same saturated-absorption-line group, the noise is not identified to be the saturated absorption line and the laser oscillation frequency can be locked to the desired one of (i.e. target) saturated absorption lines.
In order to check whether or not a laser beam of a desired laser oscillation frequency is outputted in the above-described typical laser-frequency stabilizer 100, a check process has to be conducted using a system as shown in FIG. 7 after the frequency locking process of the control unit 31.
FIG. 7 shows an outline of a system arrangement for checking the oscillation frequency of the laser beam.
As shown in FIG. 7, a reference-laser-beam source 200 configured to output a reference laser beam L5 having a known oscillation frequency is provided independently of the laser-frequency stabilizer 100.
Then, the optical axes of the reference laser beam L5 and the laser beam L3 outputted from the laser-frequency stabilizer 100 are coaxially arranged using, for instance, an optical axis adjustment reflector mirror 201 and a beam splitter 202 as shown in FIG. 7 before the reference laser beam L5 and the laser beam L3 are incident on the high-speed light detector 203. In the high-speed light detector 203, a frequency difference (beat frequency) between the laser beam L3 and the reference laser beam L5 is detected, and the beat frequency is measured using a frequency counter 204 based on beat signals outputted by the high-speed light detector 203. By determining whether or not the measured beat frequency is the same as the frequency difference between a target frequency and the frequency of the reference laser beam L5, it can be determined whether or not the laser beam L3 outputted from the laser-frequency stabilizer 100 is the laser beam of the desired oscillation frequency.
However, in order to provide the system shown in FIG. 7, the reference-laser-beam source 200, the high-speed light detector 203 and the frequency counter 204 for measuring the beat frequency, and the optical axis adjustment reflector mirror 201 and the beam splitter 202 for coaxially arranging the optical axes of the laser beam L3 and the reference laser beam L5 are required, thereby complicating the system arrangement and increasing the system cost.